The present invention relates to a power supply for converting A.C. to D.C. and is especially useful for driving a flash lamp used to illuminate an original document in an electrostatic copying machine.
In an electrostatic copying machine a light image is radiated onto a charged, photoconductive member to form an electrostatic image which is developed and transferred to a copy sheet to provide a permanent copy. Various copying machines have been developed which use as the photoconductive member an endless belt which has flat runs. The belt configuration allows the entire light image to be radiated onto the belt at one time as opposed to slit exposure used in copying machines having photoconductive drums.
Since it is undesirable to stop the belt and hold it stationary during exposure, since this would lower the copying speed, illumination of the original document for exposure is performed by a flash lamp or tube which provides an intense flash of light for a very short duration. This has the effect of freezing the image relative to the belt and making it possible to expose the belt while it is moving at constant speed.
The power supply used to drive the flash tube generally comprises a step-up transformer to provide a high A.C. output from standard line voltage, a rectifier for converting the output of the transformer into pulsating D.C. and a filter capacitor for smoothing and storing the pulsating D.C. to produce approximately constant D.C. for application to the flash tube. A switch connected between the A.C. line power source and the transformer opens when the voltage across the flash tube reaches a predetermined high value to prevent applying an excessively high voltage to the flash tube. At the desired time, a signal is applied to a trigger electrode of the flash tube which causes the capacitor to discharge through the tube, which fires and emits light.
Upon firing of the tube, the voltage across the capacitor drops to substantially zero and the switch is closed to recharge the capacitor. At the time the switch is closed a very high surge current flows through the various elements of the power supply. If no means are provided to limit the surge current, the transformer, rectifier, capacitor, switch etc. must have excessively large current capacities to withstand the surge current. Such high current components are undesirable from the standpoints of size and cost.
It has been known to provide a current limiting element between the switch and the rectifier, such as between the transformer and the rectifier. The limiting element may be a resistor, capacitor or inductor. The resistor does not affect the power factor of the power supply but dissipates current at all times and produces heat. This constitutes a waste of energy and requires that the transformer provide increased step-up voltage and current to compensate for the loss.
Inductors and capacitors do not actually dissipate power but introduce a phase shift into the power supply which substantially reduces the power factor. The reduced power factor means that the various components of the power supply must have an unnecessarily large apparent power (volt-ampere product) capacity since the voltage and current have been shifted out of phase by the current limiting capacitor or inductor.